Seismic logging tools are know for use in performing vertical seismic profile surveys from boreholes to evaluate the surrounding underground formations. The tools can be single sondes or multi-shuttle tools comprising a number of similar sondes (shuttles) linked together by means of a cable and logged through a borehole while seismic signals are generated at the surface. At each measurement level, each shuttle is anchored to the wall of the borehole during a measurement operation by means of an anchoring arm. The sensors (geophones or geophone accelerometers) are mounted in packages which are located in the tool body in such a way that they are isolated from the vibrational behaviour of the tool when anchored in the borehole. This is typically done by spring-mounting the sensor package in the tool and clamping the tool body securely to the borehole wall (by anchoring the tool as described above) when measurement are to be made. One such seismic logging tool is Schlumberger's Combinable Seismic Imager tool (CSI). Aspects of the CSI are described in U.S. Pat. No. 4,563,757; U.S. Pat. No. 4,575,831; U.S. Pat. No. 4,987,969; and U.S. Pat. No. 5,200,581. In particular, patents '969 and '581 relate in particular to the sensor package and deployment. Schematic figures from the patents is shown in FIGS. 1a-b and 2a-b. FIGS. 1a and 1b show a sensor package 30 which is housed in a cavity in the tool body 21. The package is connected to the tool body by a resilient suspension formed from two coil compression springs 40, 50 which locate around centring studs 40a, 50a, 40b, 50b, on the package 30 and tool body 21. Retraction actuators 41, 51 pass through the coil springs and engage in bores in the package 30. In the retracted position, the actuators are withdrawn such that the flared ends engage in the bores and pull the package into the cavity against the action of the springs. Thus the package is held within the cavity and is protected from mechanical damage when running in or pulling out of hole. When the tool is deployed and the tool body anchored against the borehole wall, the actuators are released and the package is pressed against the borehole wall by the action of the springs.
FIGS. 2a and 2b show an alternative system which still uses springs 140, 150 and centring studs 140a, 140b, 150a, 150b, but the actuators and bores are replaced by a system of a rod 160 and rockers 143, 153. Pulling on the rod causes the rockers to retract the sensor package into the tool, while releasing the rod allows the springs to force the package out of the cavity as before.
In both cases, the tool includes an active system for retracting the sensor package into the tool body. This adds to the size, weight and complexity of the sonde which can be significant where a number of sondes are to be linked together as in a shuttle tool.
Other prior art systems are described in U.S. Pat. No. 4,874,060 and U.S. Pat. No. 4,811,814; Borehole seismic shuttle tools currently used also include the SST500 of Compagnie General de Geophysique and the seismic shuttle tools of VNIIGIS/OZGA.
The present invention has as its object a tool in which the use of a retracting mechanism is avoided while still providing for secure location of the sensor package in the tool.